General Bike Maintenance

Hi everyone, hoping someone can point me oi the right direction, I have an old (87) GB500TT which has a bit of an issue with cutting out at the lights when idling fine (1400rpm). Weird, it all of a sudden just stops!! It fires up fine but usually find it is a bit worse when very warm or has had an enthusiastic ride, anyone come accross this before? I remember hearing about a mod when they were new from someone but cannot recall details, thanks